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Thread: Best way to Combine Multi-Mode Output

  1. #1
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    Default Best way to Combine Multi-Mode Output

    I am looking into combining two 445s, into one output.

    I just need some opinions on the best way to do it. I've found group buys that would allow me to use a coated PBS cube, which would give me an output in the shape of a (+).

    I've also considered the knife edging technique. It would allow for a better looking beam profile that sort of looks like (ll).

    Which beam profile would allow for best beam specs? Low divergence would be nice, but a smaller beam diameter would also be nice. I'm just having trouble deciding since these diodes put out such a long strip instead of a round spot. Just not used to working with a particularly fast axis, so I'm looking to avoid having to correct anything right off the diode.

    So is PBS combination the better method for better beam specs in this multimode situation? Or is Knife Edge the way to go? Would the Laserman532 setup be user friendly enough for a beginner, to beam alignments?

    Would it be advantageous in either situation, to expand the beam and re-collimate it after the combination, to help with divergence?

    Note: This would be the first time I've combined beams.

    Thanks Guys

    -Tyler


  2. #2
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    Lightbulb

    How much do you have to spend on optics? My thought is to knife edge two H polarized beams together, one on top of the other, then knife edge those two beams in half, taking half the beam(s) to a waveplate to make two small V polarized beams; PBS those with the two small H polarized beams into a # or small square.
    Love, peace, and grease,

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  3. #3
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    Default

    Quote Originally Posted by allthatwhichis View Post
    How much do you have to spend on optics? My thought is to knife edge two H polarized beams together, one on top of the other, then knife edge those two beams in half, taking half the beam(s) to a waveplate to make two small V polarized beams; PBS those with the two small H polarized beams into a # or small square.
    I've seen this suggested before, and I can't wait to see an example of this in operation!

  4. #4

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    allthatwhichis's idea is pretty nice, although I think that the small dimension of the beam is the fast axis. I was also under the assumption that a waveplate simply rotated the polarization of the beam, not the pattern of the beam stripe itself.

    I had similar ideas of just chopping the long dimension of the beam in half and recombining into a 2mm square beam instead of a 4mm by 2mm beam, but mccarrot reminded me that the small dimension of the beam was the fast axis. Therefore I would have been cutting the slow axis in half, resulting in a beam that would be an even thinner stripe at long distances.

    So I'm thinking that a knife-edged setup will provide the best beam specs as you would have your fast axis on a single axis of space whereas with a PBS cube combined setup you're rotating the fast axis of one diode 90-degrees, therefore placing the high divergence of that fast axis in both the horizontal and vertical directions.

    I can't remember the typical fast axis to slow axis divergence ratio for these diodes, but say that you have a fast axis divergence of 2.5mRad and a slow axis divergence of 0.5mRad. With a knife edge setup, you'll retain the same divergence figures as the two diodes' fast axes are parallel to each other.

    Therefore you'd have a divergence of 2.5mRad by 0.5mRad with a knife-edged setup or a 2.5mRad by 2.5mRad divergence with a PBS setup.

    Although I've never seen either in person, on paper the squarer beam of a knife-edged setup looks better than the cross shape beam, in my opinion. As the beams individually diverge they will converge together into a solid rectangular/square beam. With a PBS cube setup, both axes diverge at the same rate so you're always stuck with the cross shape.

    Furthermore you'll loose more power by using a PBS cube if you use high quality mirrors.

    For more fun you could take two knife-edged setups and combine them using a PBS cube for 3-4 watts in the same size beam as the dual knife-edged setup.

    Unfortunately, you'd increase the vertical divergence in doing so and wind up with square divergence figures again.

    Hope that helps; someone please correct me if I'm wrong.

    - Kyle

  5. #5
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    Default

    Quote Originally Posted by Kyle View Post
    allthatwhichis's idea is pretty nice, although I think that the small dimension of the beam is the fast axis. I was also under the assumption that a waveplate simply rotated the polarization of the beam, not the pattern of the beam stripe itself.

    I had similar ideas of just chopping the long dimension of the beam in half and recombining into a 2mm square beam instead of a 4mm by 2mm beam, but mccarrot reminded me that the small dimension of the beam was the fast axis. Therefore I would have been cutting the slow axis in half, resulting in a beam that would be an even thinner stripe at long distances.

    So I'm thinking that a knife-edged setup will provide the best beam specs as you would have your fast axis on a single axis of space whereas with a PBS cube combined setup you're rotating the fast axis of one diode 90-degrees, therefore placing the high divergence of that fast axis in both the horizontal and vertical directions.

    I can't remember the typical fast axis to slow axis divergence ratio for these diodes, but say that you have a fast axis divergence of 2.5mRad and a slow axis divergence of 0.5mRad. With a knife edge setup, you'll retain the same divergence figures as the two diodes' fast axes are parallel to each other.

    Therefore you'd have a divergence of 2.5mRad by 0.5mRad with a knife-edged setup or a 2.5mRad by 2.5mRad divergence with a PBS setup.

    Although I've never seen either in person, on paper the squarer beam of a knife-edged setup looks better than the cross shape beam, in my opinion. As the beams individually diverge they will converge together into a solid rectangular/square beam. With a PBS cube setup, both axes diverge at the same rate so you're always stuck with the cross shape.

    Furthermore you'll loose more power by using a PBS cube if you use high quality mirrors.

    For more fun you could take two knife-edged setups and combine them using a PBS cube for 3-4 watts in the same size beam as the dual knife-edged setup.

    Unfortunately, you'd increase the vertical divergence in doing so and wind up with square divergence figures again.

    Hope that helps; someone please correct me if I'm wrong.

    - Kyle
    Thanks for the Replys!
    I think allthatwhichis is talking about rotating polarization, after the beam has been split into some rough form of a square. His idea is very intriguing... and pricey sounding. I'll admit, I'm on a budget. I have a few OD7 goggles to save up for as well.

    I'm not too worried about correcting the initial output I suppose, I'm mainly interesting in finding out which beam profile is more desirable. I've never read of beams that are shaped like the "blue" cross symbol.

    The least amount that I can spend on optics, and optics mounts would be desired. Anyone have any experience with both methods?
    Last edited by Meatball; 06-22-2010 at 19:30.


  6. #6

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    I thought that allthatwhichis was saying that one could stack two beams with the long dimension of the beam in the horizontal direction, split the length with a mirror and send one half through a waveplate and the other half directly through a PBS cube. As far as I know, waveplates only rotate the polarization of the beam. Therefore, recombining the two halves with a PBS cube would indeed result in a nearly square beam with the two beams on top of each other, except that the width of the beam would be half that of the original beam. The beam profile would look more like (=) instead of (#) as a waveplate does not rotate the beam itself, only the polarization of the light within the beam. However, the two lines stacked on top of each other would be half the length they were originally.

    Ultimately you're only making the long part of the beam, the slow axis, half as long. Since the divergence of both axes will remain the same, at long distances you'll wind up with a very tall beam as the height of the two beams has increased much more rapidly than their width.

    With a PBS setup you'll have at least one optic mount in addition to the diode block so that you can hold/position your PBS cube. However it will be difficult to obtain maximum power through the cube as you won't be able to adjust the position of either beam, just the cube itself. It would be much easier to align if you mount both diodes parallel to each other and bounce of the beams into the PBS cube while placing the directly in front of the beam you'd like to transmit through the cube.

    With this design you're looking at two optic mounts, the diode mount, one mirror, and one PBS cube, whereas if you went with a knife-edged setup you'd replace the expensive PBS cube with another mirror. PBS cubes will run at least $50 or so, while a decent mirror should run less than half that.

    I'd wait to see if a group buy does form for dielectric mirrors designed for 445nm. These would likely achieve less than one percent loss from the mirror.

    I think mccarrot said that the PBS cubes he sells have about 3% total loss at 445nm, and cost 42 euro, or about $52 plus shipping at the current exchange rate.

    - Kyle

  7. #7

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    I think there's some terminology getting mixed here. "Fast" and "slow" describe the relative divergences of the 2 axes, so the fast-diverging direction is the wide direction in the far-field.

    Another terminology that works well with respect to the diode itself is transverse vs. lateral, with lateral being in the plane of the diode itself, and transverse being the direction normal to the diode, as it sits in the can. In this case, the wide direction of the farfield corresponds to the transverse direction, and the narrow (and multimode) direction corresponds to the lateral direction.

  8. #8

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    Quote Originally Posted by pullbangdead View Post
    I think there's some terminology getting mixed here. "Fast" and "slow" describe the relative divergences of the 2 axes, so the fast-diverging direction is the wide direction in the far-field.
    There's a bit of a catch here. That definition is based on a diode with no collimating optics. The axis which diverges most rapidly coming out of the die is the fast axis. However, if you collimate with a single lens, the fast and slow axes will switch in the far field! Why? The beam is wide in the fast axis as it exits the collimator, so its divergence in that axis is low. The beam width in the slow axis is much narrower coming out of the collimator, so divergence is higher in that axis. Thus, in the far field, the more divergent axis is the diode's SLOW axis! Using an anamorphic prism pair or a cylindrical lens telescope to adjust the beam's aspect ratio will eliminate this effect.
    Last edited by mpoulton; 06-24-2010 at 09:03. Reason: HTML tag problem

  9. #9
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    Default

    Quote Originally Posted by mpoulton View Post
    There's a bit of a catch here. That definition is based on a diode with no collimating optics. The axis which diverges most rapidly coming out of the die is the fast axis. However, if you collimate with a single lens, the fast and slow axes will switch in the far field! Why? The beam is wide in the fast axis as it exits the collimator, so its divergence in that axis is low. The beam width in the slow axis is much narrower coming out of the collimator, so divergence is higher in that axis. Thus, in the far field, the more divergent axis is the diode's SLOW axis! Using an anamorphic prism pair or a cylindrical lens telescope to adjust the beam's aspect ratio will eliminate this effect.
    Thats a good explantion, I took my 445 diode and demonstrated to myself what you are describing with a single lens and it all makes sense now!

    @meatball
    If you havnt combined beams before I would suggest waiting for Dr. Lava to release his kit. A lot of people are waiting on it but it should be a very high quality and affordable solution. (perfection takes time)

  10. #10
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    Quote Originally Posted by MisterWilling View Post
    Thats a good explantion, I took my 445 diode and demonstrated to myself what you are describing with a single lens and it all makes sense now!

    @meatball
    If you havnt combined beams before I would suggest waiting for Dr. Lava to release his kit. A lot of people are waiting on it but it should be a very high quality and affordable solution. (perfection takes time)

    @ meatball
    or, you could start today...tomorrow may never come

    @misterwilling
    Im sure DrLava Kit will be spectacular...but even an artist rendition would be nice...a model, a price...a SOMETHING until then...I think I make a pretty nice solution for hobbiest. EDIT...that ships today!
    Last edited by Laserman532; 06-24-2010 at 09:22.
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